This invention relates to display devices.
One specific class of display devices is those that use an organic material for light emission. Light-emissive organic materials are described in PCT/WO90/13148 and U.S. Pat. No. 4,539,507, the contents of both of which are incorporated herein by reference. The basic structure of these devices is a light-emissive organic layer, for instance a film of a poly(p-phenylenevinylene (“PPV”), sandwiched between two electrodes. One of the electrodes (the cathode) injects negative charge carriers (electrons) and the other electrode (the anode) injects positive charge carriers (holes). The electrons and holes combine in the organic layer generating photons. In PCT/WO90/13148 the organic light-emissive material is a polymer. In U.S. Pat. No. 4,539,507 the organic light-emissive material is of the class known as small molecule materials, such as (8-hydroxyquinoline)aluminum (“Alq3”). In a practical device one of the electrodes is typically transparent, to allow the photons to escape the device.
FIG. 1 shows the typical cross-sectional structure of an organic light-emissive device (“OLED”). The OLED is typically fabricated on a glass or plastic substrate 1 coated with a transparent first electrode 2 such as indium-tin-oxide (“ITO”). Such coated substrates are commercially available. This ITO-coated substrate is covered with at least a layer of a thin film of an electroluminescent organic material 3 and a final layer forming a second electrode 4, which is typically a metal or alloy. Other layers can be added to the device, for example to improve charge transport between the electrodes and the electroluminescent material.
If light that is incident on the display can be reflected back towards a viewer, especially from the region of pixels that are intended to appear dark, then the apparent contrast between the pixels of the display can be reduced. This reduces the effectiveness of the display.